The present invention relates to a cellulose acetate semipermeable membrane used for separation and concentration treatment in various fields besides water treatments such as drinking water treatment, sewage treatment and waste water treatment, and a process for producing the cellulose acetate semipermeable membrane.
Heretofore, a cellulose acetate membrane has been used as a material of separation membrane for various kinds of water treatment and for medical use in blood dialysis etc., for the reason of hydrophilicity and a little decrease in filtration rate, and widely used at present particularly in the form of a reverse osmosis membrane. For demand in practical use, the reverse osmosis membrane made of cellulose acetate is often constructed such that a separation active layer in the membrane is made very dense with a membrane pore diameter of 0.001 xcexcm or less, but there arise various problems attributable to the membrane structure.
For example, JP-B 58-24164 discloses a reverse osmosis membrane made of cellulose acetate having a dense layer. However, since this reverse osmosis membrane has a dense layer, the operating pressure should be kept at high pressure as high as 1 MPa or more to increase the filtration rate, which however leads to problems including not only an increase in energy cost but also a decrease in water permeation rate and mechanical breakage of the membrane due to the compaction and densification of the membrane during filtrating operation. For application of this membrane t o a blood dialysis membrane, the membrane thickness has been rendered thin to raise the plasma separation rate, but this results in the general problem of easy breakage at low pressure.
The reverse osmosis membrane made of cellulose acetate has another problem that pinholes are generated due to the presence of micro-voids. JP-B 60-43442 discloses a method for suppressing the generation of pinholes and for improving membrane strength. However, this prior art does not solve the problem of a decrease in filtration rate during practical use, because the membrane has a substantially uniform structure containing no void layer.
JP-A 6-343842 and JP-A 8-108053 disclose a cellulose acetate hollow fiber separation membrane having a three-dimensional network-like part and a void part.
Another requirement of such a membrane material for various kinds of water treatment is that it is hardly degraded by microorganisms in order to suppress a decrease in its filtration ability and to increase the longevity of the membrane thus preventing an increase in operating cost.
The object of the present invention is to provide a cellulose acetate semipermeable membrane which has a high filtration rate at a low pressure, has high mechanical strength and is hardly degraded by microorganisms, and a process for producing the cellulose acetate semipermeable membrane.
Further, JP-A10-305220discloses a hollow fiber membrane fabricated by discharging a membrane-forming solution through a double pipe type spinning orifice while discharging an inside coagulating solution from a central pipe of the spinning orifice.
As a result of their eager study on the structure of cellulose acetate serving as the starting material for production of semipermeable membranes, the present inventor have found that 3 elements relating to the structure described above, which are specified by connecting them with one another, can act synergistically to achieve the object described above, thus completing the present invention.
That is, the present invention provides a cellulose acetate semipermeable comprising a cellulose acetate produced from cellulose having an xcex1-cellulose content of not less than 99% by weight, and having a 6 wt/vol % viscosity at 25xc2x11xc2x0 C. of 20 to 220 mPaxc2x7s and an acetylation degree of 58 to 62%. Also, it provides use of the cellulose acetate as a semipermeable membrane or a hollow fiber membrane.
Preferably, the semipermeable membrane is a hollow fiber membrane.
Further, the present invention provides a cellulose acetate hollow fiber membrane, wherein the thickness of the hollow fiber membrane is 100 to 400 xcexcm, the cross-section of the hollow fiber membrane is composed of a three-dimensional network-like part and a void part, the void part is positioned inside 10 xcexcm or more from both internal and external surfaces of the membrane, the area occupied by the void part is in the range of 5 to 60% of the total cross-sectional area of the membrane, a dense layer having a surface average pore size of 0.001 to 0.05 xcexcm exists on each of the internal and external surfaces of the hollow fiber, and a crack-like muscular pattern (slit structure) is observed on the internal surface of the hollow fiber with an electron microscope at a magnification of xc3x9720, 000.
Preferably, the cellulose acetate is produced from cellulose having an xcex1-cellulose content of not less than 99% by weight. Preferably, the 6% viscosity thereof is 50 to 200 mPaxc2x7s, and the acetylation degree thereof is 60.5 to 61.5%.
Further preferably, the pure water permeation rate is not less than 500 l/(m2xc2x7h), and the tensile strength at break is not less than 4 MPa.
Further, the present invention provides a process for producing a cellulose acetate semipermeable membrane, wherein a solution of the above-described cellulose acetate dissolved in a water-soluble, organic polar solvent, for example, dimethyl sulfoxide, N-methyl-2-pyrrolidone or dimethylacetamide, is used to produce the membrane.
Preferably, the solution is formed into a hollow fiber membrane by applying a wet or dry wet spinning process using a double pipe type spinning orifice.
Preferably, at least one selected from compounds containing metallic elements of the I to III groups of the periodic table, ethylene glycol and polyethylene glycol are dissolved with cellulose acetate.
Further, the present invention provides a process for producing a cellulose acetate hollow fiber membrane wherein the thickness of the membrane is 100 to 400 xcexcm, the cross-section of the hollow fiber membrane is composed of a three-dimensional network-like part and a void part, the void part is positioned inside 10 xcexcm or more from both internal and external surfaces of the membrane, the area occupied by the void part is in the range of 5 to 60% of the total cross-sectional area of the membrane, a dense layer having a surface average pore diameter of 0.001 to 0.05 xcexcm exists on each of the internal and external surfaces of the follow fiber, and a crack-like muscular pattern (slit structure) is observed on the internal surface of the hollow fiber with an electron microscope at a magnification of xc3x9720,000, comprising dissolving a cellulose acetate in a water-soluble, organic polar solvent, and then discharging the resulting membrane-forming solution from a double pipe nozzle while discharging an inside coagulating solution from the inner pipe of the double pipe to coagulate the membrane-forming solution in a coagulation bath.
Preferably, the linear velocity of the inside coagulating solution discharged is 8 times or more as high as the linear velocity of the membrane-forming solution discharged.
Preferably, the membrane-forming solution contains a compound containing metallic elements of the I to III groups of the periodic table in an amount of 0.2 to 1.5% by weight therein.
In addition, the present invention provides use, as water treatment or separation and concentration treatment, of a cellulose acetate hollow fiber membrane in which the thickness of the hollow fiber membrane is 100 to 400 xcexcm, a cross-section of the hollow fiber membrane consists of a three-dimensional network-like part and a void part, the void part is positioned 10 xcexcm or more inside from both internal and external surfaces of the membrane, the area occupied by the void part is in the range of 5 to 60% of the total cross-sectional area of the membrane, each of the internal and external surfaces of the follow fiber has a dense layer having a surface average pore size of 0.001 to 0.05 xcexcm, and a crack-like muscular pattern (slit structure) is observed on the internal surface by observation under an electron microscope at a magnification of xc3x9720,000.
Further additionally, the present invention provides a method of conducting water treatment or separation and concentration treatment using a cellulose acetate hollow fiber membrane in which the thickness of the hollow fiber membrane is 100 to 400 xcexcm, a cross-section of the hollow fiber membrane consists of a three-dimensional network-like part and a void part, the void part is positioned 10 xcexcm or more inside from both internal and external surfaces of the membrane, the area occupied by the void part is in the range of 5 to 60% of the total cross-sectional area of the membrane, each of the internal and external surfaces of the hollow fiber has a dense layer having a surface average pore size of 0.001 to 0.05 xcexcm, and a crack-like muscular pattern (slit structure) is observed on the internal surface by observation under an electron microscope at a magnification of xc3x9720,000.
The cellulose acetate semipermeable membrane of the present invention exhibits a high filtration rate and high mechanical strength and is hardly degraded by microorganisms because the cellulose acetate as the starting material satisfies the prescribed 3 elements i.e. xcex1-cellulose content, 6% viscosity at 25xc2x11xc2x0 C. and acetylation degree. Further, it is also excellent in that the procedure of forming the membrane is easy.
The cellulose acetate hollow fiber membrane of the present invention is highly reliable, since it is excellent in water permeability, has high mechanical strength and shows a suppressed generation of membrane defects such as pinholes.
Further, the membrane is preferable in the case of the following conditions:
Size of the void part: 10 to 200 xcexcm.
Size of the slit: 0.05 to 1.0 xcexcm length,
0.005 to 0.2 xcexcm width,
slit length/slit width=5 to 50,
at least one slit per xcexcm2 is observed under an SEM at a magnification of xc3x9720, 000.
Hereinafter, with the explanation of an example of the process for producing a cellulose acetate semipermeable membrane according to the present invention, the cellulose acetate semipermeable membrane is illustrated.
Cellulose acetate as the raw material of the cellulose acetate semipermeable membrane has a specific structure satisfying the prescribed 3 elements as described below, but the cellulose itself as the starting material is not particularly limited, and plant cellulose such as industrial pulps, linters etc., bacterial cellulose and regenerated cellulose such as rayon may be used. Among these, cotton linters are preferable.
The cellulose acetate is the one having an xcex1-cellulose content of not less than 99% by weight, preferably not less than 99.5% by weight. If the content of xcex1-cellulose is not less than 99% by weight, a gel content not dissolved in the membrane-forming solution is reduced so that the generation of pinholes can be suppressed and the strength of the membrane can also be increased.
The 6% based wt/vol, herein after, viscosity at 25xc2x11xc2x0 C. of the cellulose acetate is 20 to 220 mPaxc2x7s, preferably 50 to 180 mPaxc2x7s. When the 6% viscosity is within the range defined above, the procedure of forming the membrane is facilitated so that the membrane can be produced even when the temperature of the membrane-forming solution is kept at relatively low temperature (100xc2x0 C. or less).
Further, the cellulose acetate is the one having acetylation degree of 58 to 62%, preferably 60 to 62%, particularly preferably 60.5 to 61.5. When the acetylation degree is within the range described above, the resulting membrane is hardly degraded by microorganisms, thus prolonging duration of the membrane and improving the spinning properties thereof. The acetylation degree is determined by the measurement method prescribed in Examples.
The cellulose acetate used in the present invention is preferably the one wherein the number of insolubles with a particle diameter of 3 to 100 xcexcm per mg cellulose acetate is preferably 10 or less, particularly preferably 5 or less. When the number of insolubles is 10 or less, it is possible to prevent occurrence of problems such as breakage of the membrane during membrane manufacturing or generation of pinholes by preventing formation of the membrane structure via phase conversion. Accordingly, in pre-treatment prior to membrane-forming, insolubles in the cellulose acetate are preferably removed by filtration under a pressure through a solvent-resistant filter having a pore diameter of 10 xcexcm or less, preferably 0.5 to 5 xcexcm, such as a sintered metal filter, a filter paper, a filter cloth, a PTFE membrane filter etc.
In the present invention, the above-mentioned cellulose acetate is dissolved in dimethyl sulfoxide, N-methyl-2-pyrrolidone or dimethylacetamide to prepare a membrane-forming solution.
As the solvent, the above-mentioned 3 organic solvents maybe used alone or in combination thereof as necessary. Along with these solvents, 1,4-dioxane, N,N-dimethylformamide, 2-pyrrolidone and/or xcex3-butyrolactone may be used in combination.
The amount of the solvent used is such that the concentration of cellulose acetate in the membrane-forming solution is made preferably 10 to 30% by weight, particularly preferably 15 to 23% by weight.
Along with the cellulose acetate, a compound containing metallic elements of the I to III groups of the periodic table may be dissolved therein. The compound includes at least one selected from acetates, halides such as chloride etc., nitrates, thiocyanates and hydrates of alkali metals such as Li, Na and K or alkaline earth metals such as Mg and Ca. Among these, lithium chloride, magnesium chloride, lithium acetate and magnesium acetate having high solubility are preferable. Further, a non-solvent such as ethylene glycol and polyethylene glycol may be added.
Preferably, the amount of the above-mentioned compound added is 0.05 to 5% by weight and the amount of the non-solvent added is 1 to 30% by weight to the total weight of the membrane-forming solution, in order to improve both the water permeability and strength of the membrane, and to facilitate the procedure of forming the membrane by preventing an increase in viscosity.
The process of forming the membrane varies depending on the type of the desired membrane, and the process of forming the membrane according to the type of membrane such as flat membrane, spiral, tube and hollow fiber may be applied. For example, the spinning process using a double pipe type spinning orifice as described below can be applied to the forming of the hollow fiber membrane. The spinning process may be either a wet process or a dry wet process.
In the case of the wet process, the membrane-forming solution is discharged from an outer pipe of a double pipe type spinning orifice, while an inside coagulating solution is discharged from an inner pipe to coagulate the solution in a coagulation bath. Both the temperature of the inside coagulating solution and the temperature of the coagulation bath are preferably in the range of 30 to 80xc2x0 C., to give the hollow fiber membrane having a dense layer of suitable thickness.
In the case of the dry wet process, the distance of a drying part between the discharge part of the spinning orifice and the coagulation bath is preferably 0.1 to 50 cm, particularly preferably 0.3 to 30 cm, and preferably the solution is introduced into the coagulation bath, after passing the air for 0.2 second or more at the distance mentioned above. The temperature of the coagulation bath is the same temperature range as in the wet process.
The solvent used in the inside coagulating solution or in the coagulation bath is the one which does not dissolve cellulose acetate and is compatible with the solvent used in forming the membrane, and as examples, water, ethylene glycol, polyethylene glycol etc. may be proposed. The compound used in preparing the membrane-forming solution may also be added to the inside coagulating solution or the coagulation bath.
When the cellulose acetate semipermeable membrane of the present invention is a hollow fiber membrane, the hollow fiber membrane preferably has the following structure:
The hollow fiber membrane preferably has a dense layer on the internal or external surface of the membrane and has both a three-dimensional network-like porous part having porosity and a void part in the inside of the membrane.
The dense layer is present in a depth of substantially up to {fraction (1/100)} of the membrane thickness from the internal or external surface of the membrane, and the surface average pore diameter is preferably in the range of 0.001 to 0.05 xcexcm, particularly preferably 0.005 to 0.03 xcexcm, and the numerical range corresponds to molecular weight cut-off of 10,000 to 500,000. For example, the decrease in filtration rate caused by penetration of suspending particles in a treated fluid into the inside of the membrane can be prevented by regulating the surface average pore size within the range defined above.
The three-dimensional network-like porous part is formed in the remainder of the density layer, and the average pore diameter of the void is smaller than the average pore diameter of the dense layer, and is substantially in the range of 0.05 to 1 xcexcm. When the average pore diameter is in this range, high mechanical strength and ductility can be conferred on the hollow fiber membrane.
The void part coexists with the three-dimensional network-like porous part and consists of round or elliptical voids (porosity) which are larger, substantially 10 to 200 xcexcm in size, than voids in the three-dimensional porous part and having little filtration resistance against a permeating fluid. The ratio of the area occupied by the void part to the cross-sectional area of the membrane is preferably 5 to 60%, particularly preferably 20 to 50%. When the ratio of the area occupied by the void part is within the range defined above, the filtration rate can be increased, and further mechanical strength such as tensile strength and burst pressure can be raised.
The membrane thickness is preferably 50 to 500 xcexcm, more preferably 100 to 400 xcexcm. When the membrane thickness is within the above range, the filtration rate and mechanical strength can be raised.
By constructing the hollow fiber membrane as the above structure, the filtration rate can be raised without deteriorating the mechanical strength of the membrane, as compared with a hollow fiber membrane having a gradient-type porous layer having a continuously increasing pore diameter from the surface of the membrane with the minimum pore diameter to the inside of the membrane.